1. Field of the Invention
The present invention relates to a near-field optical head for optical information recording/reproducing apparatus that records and/or reproduces the information on a high-density recording medium using near-field light.
2. Description of the Related Art
Now, most of the information recording/reproducing apparatus using light records or reproduces information on optical disks applied as information recording/reproducing medium. Especially among them, compact disk (CD) as one of the optical disk is widely used as recording medium capable of reproducing large quantity of information. Pit (recording unit of the information) having a size nearly equal to the wavelength of the laser beam used for reproducing the information and a depth about one fourth of the wavelength is formed on the surface of CD which is read using coherent light (reproducing of the information).
The optical information recording/reproducing apparatus of this kind is on the way toward the increase in capacity and the downsizing thereby requiring for ever-denser approach of the recording pit. However, in a conventional optical information recording/recording apparatus, optical lens system employed in optical microscope is generally used. This lens system can not reduce the spot diameter of the light by more than half a wavelength because of the diffraction limit of the light. Therefore, there was a limitation of the information recording density.
As an approach for breaking the limitation of the information recording density, research on the information recording/reproducing method is pursued utilizing the interaction between the near-field light emitted from a minute aperture and the minute region. By means of using near-field light, it is made possible to deal with the optical information in a region of less than a wavelength which was thought to be a limit in conventional optical lens system.
As an example of the information reproducing method using near-field light, a method is known in which near-field light is generated from a minute opening put in a proximity of the surface of the recording medium and diffracted light transformed by the interaction between the minute structure recorded as information on the surface of the recording medium is detected by photo detecting system provided separately. As an example of information recording, a method is known in which near-field light generated from a minute opening is irradiated on the surface of the recording medium to perform recording by means of changing the shape of a minute region of the recording medium or changing refractive index or light transmission factor. In this way, with the use of near-field optical head having a minute opening, it becomes possible to achieve an ever-denser approach of the recording pit that exceeds the limit of the light diffraction which conventional optical information recording/reproducing apparatus could hardly realize.
However, near-field optical head of above-mentioned kind has a low utilization factor of light and generates near-field light of quite small intensity. Therefore, in case of rapid recording and/or reproducing information, there was a problem that light with sufficient quantity can not be obtained in photo detecting system upon reproducing the information and that light with sufficient quantity can not be obtained as for the change in shape of the minute region, refractive index or light transmission factor upon recording the information. Consequently, in the case of constructing optical information recording/reproducing apparatus using near-field optical head, there was a problem that nothing but low rate recording and/or reproducing was possible.
Thus, this invention is proposed in consideration of above-mentioned related art and the object of the invention is to provide a near-field optical head capable of rapid recording and/or reproducing information by means of generating constant intensive near-field light continually.
In order to solve the aforementioned problem, the first near-field optical head according to the present invention is related to a near-field optical head for recording and/or reproducing information on recording medium using near-field light comprising a light source, a lens for condensing light from said light source, a light condensing mark having a concave or a convex shape for reflecting the light from said light source, a minute opening provided in said light condensing mark, detecting means for detecting the relative position between the light condensing point condensed by said lens and said light condensing mark, and servo means for controlling said light condensing point to follow said light condensing mark.
Consequently, according to the near-field optical head related to the first invention, by means of irradiating condensed light on the light condensing mark, the energy density of the incident light on the minute opening is increased so that the near-field light in the minute opening can be intensified. In addition, with the use of a detecting means for detecting the relative position between the light condensing point and the light condensing mark and means for controlling the light condensing point to follow the light condensing mark, the light condensing point exists always on the light condensing mark so that the light can be irradiated continually with constant quantity on the minute opening provided in the light condensing mark. As a result, intensive and constant near-field light can be generated in the minute opening at all times so that rapid recording and/or reproducing of the information can be made possible using the near-field optical head.
Further, the second near-field optical head according to the present invention is related to the first near-field optical head characterized by that said servo means comprises a lens actuator for moving said lens.
Further, the near-field optical head according to the third invention is related to the first or the second near-field optical head characterized by that said servo means comprises beam deflecting means provided between said lens and said light source.
Consequently, intensive and constant near-field light can be irradiated always on the light condensing mark because the light condensing point is controlled to follow the light condensing mark by means of the lens movement by the lens actuator or by means of the beam deflecting means. Therefore, intensive and constant near-field light can be generated in the minute opening at all times so that rapid recording and/or reproducing of the information is made possible using the near-field optical head. In addition, as the light condensing point is controlled to follow the light condensing mark, similar function can be realized even though the manufacture or the assembly accuracy of the near-field optical head may be comparatively low. Therefore, cheap near-field optical head can be provided.
Further, the fourth near-field optical head according to the present invention is related to the first to the third near-field optical head characterized by that said detecting means comprises a photo detector for detecting the reflected light from said light condensing mark.
Consequently, because the light reflected back by the light condensing mark is used, the detection of the relative position between the light condensing point and the light condensing mark can be performed simultaneously with the recording and/or the reproducing of the information. Therefore, intensive and constant near-field light can be generated in the minute opening at all times so that rapid recording and/or reproducing of the information is made possible.
Further, the fifth near-field optical head according to the present invention is related to the fourth near-field optical head characterized by that said detecting means employs knife-edge method for detecting the relative position between the light condensing point and said light condensing mark along the direction approximately in parallel with the optical axis.
Further, the sixth near-field optical head according to present invention is related to the fifth near-field optical head characterized by that said detecting means comprises a two-piece photo detector for receiving a part of said reflected light and a knife-edge for cutting off a part of said reflected light.
Further, the seventh near-field optical head according to the present invention is related to the fourth near-field optical head characterized by that said detecting means employs astigmatism method for detecting the relative position between the light condensing point and said light condensing mark along the direction approximately in parallel with the optical axis.
Further, the eighth near-field optical head according to the present invention is related to the seventh near-field optical head characterized by that said detecting means comprises a four-piece photo detector for receiving a part of said reflected light and a cylindrical lens provided between said four-piece photo detector and said lens.
Further, the ninth near-field optical head according to the present invention is related to the fourth near-field optical head characterized by that said detecting means employs beam size detecting method for detecting the relative position between the light condensing point and said light condensing mark along the direction approximately in parallel with the optical axis.
Further, the tenth near-field optical head according to the present invention is related to the ninth near-field optical head characterized by that said detecting means comprises beam splitter means for dividing said reflected light into two beams and a pair of three-piece photo detector each for receiving said split beam divided by said beam splitter into two beams.
Further, the eleventh near-field optical head according to the present invention is related to the fourth near-field optical head characterized by that said detecting means employs image rotation method for detecting the relative position between the light condensing point and said light condensing mark along the direction approximately in parallel with the optical axis.
Further, the twelfth near-field optical head according to the present invention relates to the eleventh near-field optical head characterized by that said detecting means comprises a two-piece photo detector for receiving a part of said reflected light, a cylindrical lens and a knife-edge provided between said two-piece photo detector and said cylindrical lens.
Consequently, according to the fifth to the twelfth invention, because a conventional focusing technology for optical disk is used as means for detecting the relative position between the light condensing point and the light condensing mark along the direction approximately in parallel with the optical axis, highly precise detection of the relative position is made possible despite using a simplified structure. As a result, intensive and constant near-field light can be generated in the minute opening at all times so that rapid recording and/or reproducing of the information is made possible using the near-field optical head having a structure of this kind.
Further, the thirteenth near-field optical head according to the present invention is related to the fourth near-field optical head characterized by that said detecting means employs three beam method for detecting the relative position between the light condensing point and said light condensing mark along the direction approximately perpendicular to the optical axis.
Further, the fourteenth near-field optical head according to the present invention relates to the thirteenth near-field optical head characterized by that said detecting means comprises a diffraction grating provided between said light source and said lens and a two-piece photo detector for receiving light diffracted by said diffraction grating and subsequently reflected back by said light condensing mark.
Further, the fifteenth near-field optical head according to the present invention is related to the fourth near-field optical head characterized by that said detecting means employs push-pull method for detecting the relative position between the light condensing point and said light condensing mark along the direction approximately perpendicular to the optical axis.
Further, the sixteenth near-field optical head according to present invention is related to the fifteenth near-field optical head characterized by that said detecting means comprises a two-piece photo detector for receiving a part of said reflected light.
Consequently, according to the thirteenth to the sixteenth near-field optical head, by means of utilizing conventional tracking technology for optical disk used as means for detecting the relative position between the light condensing point and the light condensing mark along the direction approximately perpendicular to the optical axis, highly precise detection of the relative position is made possible despite using a simplified structure. Therefore, intensive and constant near-field light can be generated in the minute opening at all times so that rapid recording and/or reproducing of the information is made possible using the near-field optical head having a structure of this kind.
Further, the seventeenth near-field optical head according to the present invention is related to the first to sixteenth near-field optical head characterized by that the depth of said light condensing mark is approximately between one eighth and one fourth of the effective wavelength of the light irradiated on said light condensing mark.
Consequently, the reflected light from the condensing mark interferes to each other in accordance with the variation of the relative position between the light condensing point and the light condensing mark so that the intensity distribution of the reflected light is changed. Therefore, forming the shape of the light condensing mark precisely, the relative position between the light condensing mark and the light condensing point can be detected precisely despite using a simplified structure. As a result, intensive and constant near-field light can be generated in the minute opening at all times so that rapid recording and/or reproducing of the information is made possible using the near-field optical head having a structure of this kind.
Further, the eighteenth near-field optical head according to the present invention is related to the first to seventeenth near-field optical head characterized by that said minute opening is provided on said floating slider.
Consequently, with the use of the floating slider, the relative speed between the minute opening provided on the floating slider and the recording medium with recorded information can be increased. As a result, a rapid scanning of the minute opening over the recording medium is made possible so that rapid recording and/or reproducing of the information becomes possible using the near-field optical head.